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Mechanical Properties
Necking: Diffuse Neck and Local Neck
A tensile bar increases in length as it is pulled, with a concurrent reduction in width and thickness. The cross section is rectangular in shape through uniform elongation. After uniform elongation, however, strains concentrate in the reduced section of the tensile test sample, resulting in a non-uniform section of reduced width. This region is known as a diffuse neck. The diffuse neck further accentuates and accelerates the cross-section reduction, leading to a concentration of strains within this region.
A local neck is a narrow band in the sheet metal part that is thinner than its surroundings (Figure 1). This local or through-thickness neck occurs shortly before the traditional fracture of the specimen. When the local neck begins, deformation stops in the remainder of the stamping. In a tensile bar, no deformation occurs along the width of the neck – only increased elongation and thinning. A local neck prevents a deeper section from being formed and serves as a crack initiation site. Additional loading, including fatigue loading during the life of the part, may cause a neck to progress to fracture (Figure 2). The strains resulting in a local neck are defined by the Forming Limit Curve, or FLC.
Figure 1: A local neck prevents forming a deeper section and serves as a crack initiation site.
Figure 2: Necking and fracture on a sample formed with a hemispherical dome.S-57
Traditional methods of detecting the onset of necking include tactile or visible sensing of the groove of the neck. Researchers are focusing on the use of non-contact approaches to define a neck. ISO 12004-2 calls for a polynomial fit of data outside the neck, but results from this method are a function of the order of the polynomial as well as the geometry of the tooling and the blank.
Using Digital Image Correlation allows for the detection in curvature evolution in the area that subsequently develops into a neck. The signature is detected for the many types of AHSS grades tested as well as other sheet metals, and correlates well with other methods. Citation S-57 presents an overview of the technique, with greater detail covered in Citations M-19 and S-58. Using the surface data geometry as measured by DIC to detect the true onset of necking enables better and more efficient use of AHSS grades through more reliable knowledge of their actual limits.
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Mechanical Properties
Total Elongation to Fracture
Deformation continues in the local neck until fracture occurs. The amount of additional strain that can be accommodated in the necked region depends on the microstructure. Inclusions, particles, and grain boundary cracking can accelerate early fracture. Total elongation is measured from start of deformation to start of fracture. Two extensometer gauge lengths are commonly used: A50 (50mm or 2 inches) and A80 (80 mm).
In the past, limited automation and measurement equipment necessitated the use of a non-repeatable technique defined in ASTM E8A-24 and ISO 6892-1I-7 as “elongation after fracture.” Here, the operator attempts to position the two fractured strips back together and hand measures the distance between two gauge marks on the sample.
Tensile testing combined with data acquisition systems are much more commonplace today. According to ASTM E8, the elongation at fracture shall be taken as the strain measured just prior to when the force falls below 10 % of the maximum force encountered during the test. Both elastic strains and plastic strains are included in the measurement.
Figure 1 : Total elongation is measured from start of deformation to start of fracture.
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Mechanical Properties
Forming forces need to exceed the yield strength for plastic deformation to occur and an engineered stamping to be produced. If a metal structure is loaded to a level below the yield strength, only elastic deformation occurs, and the load can be removed. With no permanent (plastic) deformation, the metal returns to its original shape.
On the stress-strain curve, yielding occurs where the initial linear region transitions to the non-linear portion. This transition does not occur always at a clearly visible well-defined point. Consistent yield strength measurement is facilitated by defining how this parameter should be determined. Two techniques are used when working with sheet metals. The most common method is to draw a line parallel to the modulus line at an offset strain of 0.2%. The intersection stress becomes what is defined at the “0.2% offset yield strength” (Figure 1). This value is referred to as Rp0.2. The second technique is drawing a vertical line at the 0.5% strain value until it crosses the stress-strain curve. This determines the “yield strength at 0.5% extension under load,” abbreviated as Rt0.5 (Figure 2). These techniques result in similar – but not identical – values for yield strength.
Figure 1: 0.2% offset yield strength, determined by offset of a line parallel to the modulus line by 0.2% strain.
Figure 2: Yield strength at 0.5% extension under load, determined by a vertical line offset from the origin by 0.5% strain
Some metals have yield point elongation (YPE) or Lüders bands. Deforming metal is locked in place by interstitial carbon and nitrogen atoms and other restrictive features of the microstructure. Load increases with little corresponding deformation – or put another way, stress increases with only an incremental increase in strain. The highest stress reached is known as the upper yield strength or upper yield point. Once a band of deformed (yielded) metal breaks free from being pinned by dislocations in the microstructure, the stress drops and there is an increase in strain. The lowest stress reached is known as the lower yield strength or lower yield point (Figure 3). The bands of deforming metal are known as Lüders bands, named after one of the people first observing the phenomenon. Lüders deformation continues at approximately a constant stress until the entire sample has yielded, and the sample begins to work harden. The total strain associated with this type of deformation is known as yield point elongation, or YPE. Stabilized, interstitial-free, vacuum degassed steel, such as ULC EDDS are not at risk of aging, and will not exhibit YPE. For those grades susceptible to YPE, leveling prior to sheet forming will minimize this tendency.
Figure 3: Defining upper yield stress, lower yield stress, and yield point elongation.
Since springback is proportional to the yield strength of the steel, knowing the yield strength allows some estimation of relative springback. Figure 4 compares mild steel, HSLA 700Y/800T, and MS 1500 AHSS having a 1400MPa yield strength. The relative magnitude of springback is indicated by the arrows shown on the horizontal axis, and reflects the increase of springback with yield strength.
Figure 4: Springback is proportional to yield strength.
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Mechanical Properties
Engineering stress-strain units are based on the starting dimensions of the tensile test sample: Engineering stress is the load divided by the starting cross-sectional area, and engineering strain is the change in length relative to the starting gauge length (2 inches, 50mm, or 80mm for ASTM [ISO I], JIS [ISO III], or DIN [ISO II] tensile test samples, respectively.)
Metals get stronger with deformation through a process known as strain hardening or work hardening. This is represented on the stress strain curve by the parabolic shaped section after yielding.
Concurrent with the strengthening as the tensile test sample elongates is the reduction in the width and thickness of the test sample. This reduction is necessary to maintain consistency of volume of the test sample.
Initially the positive influence of the strengthening from work hardening is greater than the negative influence of the reduced cross-section, so the stress-strain curve has a positive slope. As the influence of the cross-section reduction begins to overpower the strengthening increase, the stress-strain curve slope approaches zero.
When the slope is zero, the maximum is reached on the vertical axis of strength. This point is known as the ultimate tensile strength, or simply the tensile strength. The strain at which this occurs is known as uniform elongation.
Strain concentration after uniform elongation results in the formation of diffuse necks and local necks and ultimately fracture.
Figure 1: Tensile Strength is the Strength at the Apex of the Engineering Stress – Engineering Strain Curve.
